A. Field of the Invention
This invention relates to the fields of acoustic engineering and digital signal processing, and more particularly to techniques for filtering and limiting acoustic signals, so as to maintain the sound pressure levels and acoustic energy associated with such signals within certain predetermined limits.
B. Description of the Related Art
Advanced telecommunication headsets and their related circuitry typically provide some method of limiting the maximum sound level that the speakers can produce. Traditional approaches to implementing these limiting circuits have focused on limiting the peak voltage of very fast transient signals as well as limiting signals sustained above a reference level over longer time periods. These signals are electrical representations of the desired acoustic signal.
Existing products and techniques implement signal limiting using analog circuitry such as diodes to provide instantaneous peak limits on the amplifier output voltage. However, maintaining sound levels below certain predetermined limits by limiting the instantaneous peak voltage using diodes causes significant distortion over a wide range of signal levels. For example, in order to insure that an acoustic signal never exceeds a sound pressure level of 110 dB using diodes to limit the instantaneous peak voltage, non-linear effects begin at a level far below the desired limiting level. The result is a degradation of sound quality of the acoustic signal at a level where linearity is desired.
Additionally, existing analog techniques have utilized gain control circuits that respond to predetermined input signal threshold levels with a predetermined time constant for their gain adjustment. Such gain adjustments limit output voltage to a predetermined level after a time interval referred to as the attack time, once the threshold is exceeded. Threshold limits are usually based on peak value of the input signal. In one prior art example, diodes limit peak levels in less than a microsecond by simple clipping, a gain control circuit reduces peak output to a lower level after a first attack time is exceeded (tuned to the timing of speech peaks) and if the threshold continues to be exceeded after a second longer attack time (tuned to the longer periodicity of continuous tones), output signal is reduced to a still lower level. See U.S. Pat. No. 4,928,307 as an example of this technique.
Such existing methods of limiting acoustic signals rely on peak or average threshold detection over a broad frequency range. Consequently, the output level of the signal is limited according to the total acoustic energy contained in the signal. However, the level of the input signal that is uncomfortable is dependent on the frequency content of the signal. For example, low frequency signals (below 1000 Hz) can reach a higher level before the discomfort range of a listener is triggered. Normal speech is a wideband signal, with much of the frequency content below 1000 Hz. Broad band limiting of speech peaks can result in a very quiet output level when the speech is within the normal amplitude range.
Other existing sound level limiting methods provide for decreasing the sound level output in the event that a portion of the signal exceeds the average speech level, and many of these other methods provide low distortion limiting. However, these methods can fail to prevent unacceptably loud sounds from being produced by the output acoustic transducers such as speakers for short periods of time (on the order of 1 to 10 milliseconds).
Additionally, in 1999, Telstra Corporation Ltd, an Australian company, released its “TT4” specification. TT4 was developed to satisfy various regulatory agencies' desires to prevent acoustic disturbances resulting from being exposed to loud sounds.
The TT4 specification calls for frequency band specific limits in order to adequately protect users of headsets from sound levels that briefly become loud enough to trigger an acoustic disturbance. Implementing frequency dependent limits to sound pressure levels would be extremely difficult using existing analog techniques, and the TT4 specification does not provide any guidance or instruction as to how the proposed limits are to be achieved, but rather merely describes the desired results.
As an example, FIG. 8 illustrates a combination of frequency dependent limits and time domain constraints. In FIG. 8 there are three curves of sound pressure level (SPL) limits as a function of frequency. The three curves represent different measurement and time dependencies as follows. (1) The top most curve represents the maximum instantaneous SPL limits as measured with an instantaneous peak detector. These limits can never be exceeded. (2). The middle curve represents the maximum SPL levels as measured using an FFT analysis with 62.5 Hz resolution over a 16 ms window. (3) The lowest curve represents the long term or sustained limits. These sustained limits use the same 16 ms measurement technique as curve (2) and must be attained within 32 ms following an acoustic situation where the a signal continues to exceeded the limits of (2).
Thus, there is a need for a method to limit an acoustic signal without causing excessive delay or distortion to the sound produced by the acoustic signal, while ensuring that the sound produced from the signal does not exceed predetermined, frequency and time dependent limits.